Adding Large TGA Files Loses Alpha
ImageMagick can handle terabytes images assuming you have enough free disk space. If you don't, ImageMagick should complain. Try adding -limit memory 64 -limit map 64 to your command line and see if that makes a difference. You can try upgrading to the latest release, ImageMagick 6.3.0. If all else fails, post a URL to your images and post the exact command you are using and we will try to reproduce the problem.